Apparatus and methods for manufacturing thin-film solar cells

ABSTRACT

Improved methods and apparatus for forming thin-film layers of semiconductor material absorber layers on a substrate web. According to the present teachings, a semiconductor layer may be formed in a multi-zone process whereby various layers are deposited sequentially onto a moving substrate web.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority under 35 U.S.C. §119 and applicableforeign and international law of U.S. Provisional Patent ApplicationSer. Nos. 61/124,467 filed Apr. 15, 2008 and 61/124,468 filed Apr. 15,2008, which are hereby incorporated by reference in their entireties.

This application also incorporates in their entirety the followingpatents and patent applications: 6,310,281, 6,372,538, 7,194,197,11/727,975, 12/154,548, 12/154,549, and 12/154,550.

BACKGROUND

The field of photovoltaics generally relates to multi-layer materialsthat convert sunlight directly into DC electrical power. The basicmechanism for this conversion is the photovoltaic effect, first observedby Antoine-César Becquerel in 1839, and first correctly described byEinstein in a seminal 1905 scientific paper for which he was awarded aNobel Prize for physics. In the United States, photovoltaic (PV) devicesare popularly known as solar cells or PV cells. Solar cells aretypically configured as a cooperating sandwich of p-type and n-typesemiconductors, in which the n-type semiconductor material (on one“side” of the sandwich) exhibits an excess of electrons, and the p-typesemiconductor material (on the other “side” of the sandwich) exhibits anexcess of holes, each of which signifies the absence of an electron.Near the p-n junction between the two materials, valence electrons fromthe n-type layer move into neighboring holes in the p-type layer,creating a small electrical imbalance inside the solar cell. Thisresults in an electric field in the vicinity of the metallurgicaljunction that forms the electronic p-n junction.

When an incident photon excites an electron in the cell into theconduction band, the excited electron becomes unbound from the atoms ofthe semiconductor, creating a free electron/hole pair. Because, asdescribed above, the p-n junction creates an electric field in thevicinity of the junction, electron/hole pairs created in this mannernear the junction tend to separate and move away from junction, with theelectron moving toward the electrode on the n-type side, and the holemoving toward the electrode on the p-type side of the junction. Thiscreates an overall charge imbalance in the cell, so that if an externalconductive path is provided between the two sides of the cell, electronswill move from the n-type side back to the p-type side along theexternal path, creating an electric current. In practice, electrons maybe collected from at or near the surface of the n-type side by aconducting grid that covers a portion of the surface, while stillallowing sufficient access into the cell by incident photons.

Such a photovoltaic structure, when appropriately located electricalcontacts are included and the cell (or a series of cells) isincorporated into a closed electrical circuit, forms a working PVdevice. As a standalone device, a single conventional solar cell is notsufficient to power most applications. As a result, solar cells arecommonly arranged into PV modules, or “strings,” by connecting the frontof one cell to the back of another, thereby adding the voltages of theindividual cells together in electrical series. Typically, a significantnumber of cells are connected in series to achieve a usable voltage. Theresulting DC current then may be fed through an inverter, where it istransformed into AC current at an appropriate frequency, which is chosento match the frequency of AC current supplied by a conventional powergrid. In the United States, this frequency is 60 Hertz (Hz), and mostother countries provide AC power at either 50 Hz or 60 Hz.

One particular type of solar cell that has been developed for commercialuse is a “thin-film” PV cell. In comparison to other types of PV cells,such as crystalline silicon PV cells, thin-film PV cells require lesslight-absorbing semiconductor material to create a working cell, andthus can reduce processing costs. Thin-film based PV cells also offerreduced cost by employing previously developed deposition techniques forthe electrode layers, where similar materials are widely used in thethin-film industries for protective, decorative, and functionalcoatings. Common examples of low cost commercial thin-film productsinclude water impermeable coatings on polymer-based food packaging,decorative coatings on architectural glass, low emissivity thermalcontrol coatings on residential and commercial glass, and scratch andanti-reflective coatings on eyewear. Adopting or modifying techniquesthat have been developed in these other fields has allowed a reductionin development costs for PV cell thin-film deposition techniques.

Furthermore, thin-film cells have exhibited efficiencies approaching20%, which rivals or exceeds the efficiencies of the most efficientcrystalline cells. In particular, the semiconductor material copperindium gallium diselenide (CIGS) is stable, has low toxicity, and istruly a thin film, requiring a thickness of less than two microns in aworking PV cell. As a result, to date CIGS appears to have demonstratedthe greatest potential for high performance, low cost thin-film PVproducts, and thus for penetrating bulk power generation markets. Othersemiconductor variants for thin-film PV technology include copper indiumdiselenide, copper indium disulfide, copper indium aluminum diselenide,and cadmium telluride.

Some thin-film PV materials may be deposited either on rigid glasssubstrates, or on flexible substrates. Glass substrates are relativelyinexpensive, generally have a coefficient of thermal expansion that is arelatively close match with the CIGS or other absorber layers, and allowfor the use of vacuum deposition systems. However, when comparingtechnology options applicable during the deposition process, rigidsubstrates suffer from various shortcomings during processing, such as aneed for substantial floor space for processing equipment and materialstorage, expensive and specialized equipment for heating glass uniformlyto elevated temperatures at or near the glass annealing temperature, ahigh potential for substrate fracture with resultant yield loss, andhigher heat capacity with resultant higher electricity cost for heatingthe glass. Furthermore, rigid substrates require increased shippingcosts due to the weight and fragile nature of the glass. As a result,the use of glass substrates for the deposition of thin films may not bethe best choice for low-cost, large-volume, high-yield, commercialmanufacturing of multi-layer functional thin-film materials such asphotovoltaics. Therefore, a need exists for improved methods andapparatus for depositing thin-film layers onto a non-rigid, continuoussubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a thin-film photovoltaic cell, according toaspects of the present disclosure.

FIG. 2 is a schematic side elevational view showing formation of ap-type semiconductor layer within a deposition chamber.

FIG. 3 is a schematic side elevational view showing interior portions ofan apparatus for forming a p-type semiconductor layer in a multi-zoneprocess.

FIG. 4 is a perspective view showing one of the zones of FIG. 3 in moredetail.

FIG. 5 is a schematic perspective view of a monitoring station fordetecting one or more properties of a layer(s) deposited on a movingweb.

FIG. 6 is a flow chart illustrating a method for producing thin-filmsemiconductor layers.

FIG. 7 is a schematic side view of a web transport device.

FIG. 8 is a perspective cut away view of a source used to depositmaterial onto a moving web.

FIG. 9 is an isolated top view of a heating device used in the sourceillustrated in FIG. 8.

FIG. 10 is a perspective view of the heating device shown in FIG. 9.

FIGS. 11-13 are schematic top views of various source and heaterconfigurations for depositing thin-film semiconductor materials onto amoving web.

DETAILED DESCRIPTION I. Introduction

Manufacture of flexible thin-film PV cells may proceed by a roll-to-rollprocess. As compared to rigid substrates, roll-to-roll processing ofthin flexible substrates allows for the use of relatively compact, lessexpensive vacuum systems, and of some non-specialized equipment thatalready has been developed for other thin-film industries. Flexiblesubstrate materials inherently have lower heat capacity than glass, sothat the amount of energy required to elevate the temperature isminimized. They also exhibit a relatively high tolerance to rapidheating and cooling and to large thermal gradients, resulting in a lowlikelihood of fracture or failure during processing. Additionally, onceactive PV materials are deposited onto flexible substrate materials, theresulting unlaminated cells or strings of cells may be shipped toanother facility for lamination and/or assembly into flexible or rigidsolar modules. This strategic option both reduces the cost of shipping(lightweight flexible substrates vs. glass), and enables the creation ofpartner-businesses for finishing and marketing PV modules throughout theworld. Additional details relating to the composition and manufacture ofthin-film PV cells of a type suitable for use with the presentlydisclosed methods and apparatus may be found, for example, in U.S. Pat.Nos. 6,310,281, 6,372,538, and 7,194,197, all to Wendt et al., and inProvisional Patent Application Ser. No. 61/063,257, filed Jan. 31, 2008.These references are hereby incorporated into the present disclosure byreference for all purposes.

FIG. 1 shows a top view of a thin-film photovoltaic cell 10, inaccordance with aspects of the present disclosure. Cell 10 issubstantially planar, and typically rectangular as depicted in FIG. 1,although shapes other than rectangular may be more suitable for specificapplications, such as for an odd-shaped rooftop or other surface. Thecell has a top surface 12, a bottom surface 14 opposite the top surface,and dimensions including a length L, a width W, and a thickness. Thelength and width may be chosen for convenient application of the cellsand/or for convenience during processing, and typically are in the rangeof a few centimeters (cm) to tens of cm. For example, the length may beapproximately 100 millimeters (mm), and the width may be approximately210 mm, although any other suitable dimensions may be chosen. The edgesspanning the width of the cell may be characterized respectively as aleading edge 16 and a trailing edge 18. The total thickness of cell 10depends on the particular layers chosen for the cell, and is typicallydominated by the thickness of the underlying substrate of the cell. Forexample, a stainless steel substrate may have thickness on the order of0.025 mm (25 microns), whereas all of the other layers of the cell mayhave a combined thickness on the order of 0.002 mm (2 microns) or less.

Cell 10 is created by starting with a flexible substrate, and thensequentially depositing multiple thin layers of different materials ontothe substrate. This assembly may be accomplished through a roll-to-rollprocess whereby the substrate travels from a pay-out roll to a take-uproll, traveling through a series of deposition regions between the tworolls. The PV material then may be cut to cells of any desired size. Thesubstrate material in a roll-to-roll process is generally thin,flexible, and can tolerate a relatively high-temperature environment.Suitable materials include, for example, a high temperature polymer suchas polyimide, or a thin metal such as stainless steel or titanium, amongothers. Sequential layers typically are deposited onto the substrate inindividual processing chambers by various processes such as sputtering,evaporation, vacuum deposition, chemical deposition, and/or printing.These layers may include a molybdenum (Mo) or chromium/molybdenum(Cr/Mo) back contact layer; an absorber layer of material such as copperindium diselenide, copper indium disulfide, copper indium aluminumdiselenide, or copper indium gallium diselenide (CIGS); a buffer layeror layers such as a layer of cadmium sulfide (CdS); and a transparentconducting oxide (TCO) layer acting as the top electrode of the PV cell.In addition, a conductive current collection grid, usually constructedprimarily from silver (Ag) or some other conductive metal, is typicallyapplied over the TCO layer.

Although the precise thickness of each layer of a thin-film PV celldepends on the exact choice of materials and on the particularapplication process chosen for forming each layer, exemplary materials,thicknesses and methods of application of each layer described above areas follows, proceeding in typical order of application of each layeronto the substrate:

Layer Exemplary Exemplary Exemplary Method Description MaterialThickness of Application Substrate Stainless steel 25 μm N/A (stockmaterial) Back contact Mo 320 nm  Sputtering Absorber CIGS 1700 nm Evaporation Buffer CdS 80 nm Chemical deposition Front electrode TCO 250nm  Sputtering Collection grid Ag 40 μm PrintingThe remainder of this disclosure focuses on various methods andapparatus for forming a semiconductor absorber layer on an underlyingsubstrate web.

II. Absorber Layer

This section describes various general considerations regardingformation of a thin-film absorber layer on a substrate web. The absorberlayer typically is p-type semiconductor in the form ofcopper-indium-gallium-diselenide (CIGS) or its readily acceptablecounterpart, copper-indium-diselenide (CIS). Other materials, such ascopper indium disulfide or copper indium aluminum diselenide, also maybe used. These different compositions, among others, can be usedessentially interchangeably as an absorber layer in various embodimentsof the present teachings, depending on the particular properties desiredin the final product. For convenience and specificity, the remainder ofthis disclosure occasionally may refer to the absorber layer as a CIGSlayer. However, it should be understood that some or all of the presentteachings also may be applied to various other suitable absorber layercompositions.

FIG. 2 illustrates schematically a configuration for the inside of anabsorber layer deposition chamber 24 according to one embodiment of thepresent teachings. As shown schematically in FIG. 2, the absorber layeris applied within the deposition chamber, and specifically within adeposition region R of the chamber, in a multi-step process. Thedeposition region, and typically the entire deposition chamber, areevacuated to near vacuum, typically to a pressure of approximately700-2000 microtorr (μTorr). This background pressure typically isprimarily supplied by selenium gas emitted into the deposition region bya selenium delivery system, resulting in deposition of selenium onto theweb. The deposition of additional materials such as gallium, indium andcopper generally may be described as a roll-to-roll,molten-liquid-to-vapor co-evaporation process.

The strip material, or substrate web, feeds in the direction of arrow 25from a pay-out roll 60 to a downstream take-up roll 68 within chamber24. As the strip material moves through chamber 24, the p-type absorberlayer is formed on the bottom surface of the substrate web (as depictedin FIG. 2). A transport-guide structure (not shown) is employed betweenrolls 60, 68 in chamber 24 to support and guide the strip. The short,open arrow which appears at the left side of the block representation ofchamber 24 in FIG. 2 symbolizes the hardware provided for the deliveryof appropriate constituent substances to the interior of chamber 24.

Within chamber 24, and specifically within deposition region R, amolten-liquid-to-vapor co-evaporation process for establishing a p-typesemiconductor layer is performed. Chamber 24 is designed specificallyfor the creation of a CIGS layer, as opposed, for example, to a CISlayer. Accordingly, structures 70, 72, 74, 76, 78, 79 and 81 function togenerate vapors of copper (70), gallium (72), indium (74) and selenium(76, 78, 79, 81) for deposition onto the moving substrate web.Structures 70-81 form the bulk of the vapor-deposition-creating system,generally indicated at 83, of the present embodiment. The vapordeposition environment created in deposition region R may provide acontinuum of evaporant fluxes. Within region R, effusion fluxes may beheld approximately constant, and by translating the substrate web overthe sources, the substrate may encounter a varying flux of materialspecifically designed to achieve optimum performance in the CIGS layer.

Blocks 70, 72 and 74, which relate to the vapor-delivery of copper,gallium and indium, respectively, represent heated effusion sources forgenerating plumes of vapor derived from these three materials. Each ofthese effusion sources may include: (1) an outer thermal control shield;(2) a boat, reservoir, or crucible containing the associated moltencopper, gallium, or indium; (3) a lid that covers the associated caseand reservoir, and that contains one or more vapor-ejection nozzles (oreffusion ports) per crucible to assist in creating vapor plumes; and (4)a specially designed and placed heater located near the effusion ports,or in some embodiments formed integrally with the ports.

Structures 76, 78, 79 and 81 represent portions of a selenium deliverysystem that creates a background selenium gas pressure in some or allparts of the deposition region. A selenium delivery system may deliverselenium directly through one or more orifices in a local Se source.Alternatively, in the embodiment of FIG. 2, circles 76, 78, 79, 81represent end views of plural, laterally spaced, generally parallelelongate sparger tubes (or fingers) that form part of a manifold thatsupplies, to the deposition environment within chamber 24, a relativelyevenly volumetrically dispersed selenium vapor. Each tube has one ormore linearly spaced outlet orifices, each orifice having a diameter ofapproximately one millimeter (1.0 mm). The delivered selenium vapor maybe derived from a single pool, site, or reservoir of selenium, whichtypically vaporizes within the reservoir through sublimation. Theselenium delivery system may be configured to provide any suitableselenium pressure within the deposition region, which in mostembodiments will fall within the range of 0.7-2.0 millitorr.

The processing rate using a roll-to-roll deposition approach is limitedonly by the web translation rate through the deposition region, and bythe web width. The web translation rate is set by the minimum timerequired for sufficient film deposition, which is determined by thedetails of the reactions that occur inside the deposition region. Themaximum web width is limited by the requirement of sufficiently uniformcomposition and thickness across the width and, as a practical matter,also may be limited by the availability of sufficiently wide rolls ofsuitable substrate material, such as 25 μm-thick stainless steel. Somevacuum coating techniques, including evaporative techniques used forCIGS deposition and described in the present disclosure, rely onevaporation sources that use arrays of orifices, or effusion ports,arranged to provide sufficiently uniform deposition. Depositionuniformity across the width of the web (concurrent with sufficientmaterial deposition) can be achieved if the effusion ports are spacedacross the web width, and if the mass flow of each effusion port iswell-controlled.

The mass flow rate from an evaporation source typically is a sensitivefunction of temperature inside the effusion source near the effusionport. Therefore, for a given geometry and configuration of effusionports, the flow rate generally is controlled through careful control ofthe temperature at each port and/or effusion source. The dependence offlow rate on temperature and other factors can be understood from thewell-developed theory of low-pressure gas flow through an orifice, andgenerally can be predicted to within 5 or 10% based on the theory.Specifically, within a vacuum there are three regimes in which lowpressure gas flow occurs: (1) the free molecular regime, (2) thetransitional flow regime and (3) the laminar or full viscous regime. Inqualitative terms, the free molecular regime describes gas flow in whichgas phase collisions are rare enough that only molecule-wall collisionsare significant. Transitional flow describes a situation wheremolecule-molecule collisions occur frequently enough to affect the flowbehavior, but do not occur frequently enough to be described accuratelyby the full viscous flow model that would be used at or near atmosphericpressure.

The determination of the applicable flow regime is achieved bycalculating the Knudsen number:

$\begin{matrix}{{Kn} = \frac{\lambda}{\Gamma}} & ( {{eqn}.\mspace{14mu} 1} )\end{matrix}$where λ is the mean free path and Γ is the orifice radius. If Kn>1, thesystem is in the free molecular regime and the mass flow rate isdescribed by the following equation:

$\begin{matrix}{F_{eff} = {{\pi\Gamma}^{2}{K( \frac{M}{2\;\pi\;{RT}} )}^{\frac{1}{2}}( {p_{1} - p_{2}} )}} & ( {{eqn}.\mspace{14mu} 2} )\end{matrix}$where F is the mass flow rate through the orifice, M is the molecularweight of the gas molecules, R is the ideal gas constant, T is thetemperature, and p₁ and p₂ are the pressures on either side of theorifice. K is an empirically determined constant which is a function ofthe aspect ratio (L/Γ, where L is the orifice length) of the orifice.For L/Γ less than 1.5, K is given by

$\begin{matrix}{K = \frac{1}{1 + {0.5\frac{L}{\Gamma}}}} & ( {{eqn}.\mspace{14mu} 3} )\end{matrix}$For L/Γ>1.5,

$\begin{matrix}{K = \frac{1 + {0.4( \frac{L}{\Gamma} )}}{1 + {0.95( \frac{L}{\Gamma} )} + {0.15( \frac{L}{\Gamma} )^{2}}}} & ( {{eqn}.\mspace{14mu} 4} )\end{matrix}$

In the case of 0.01<Kn<1, there are two equations which must be solvedfor both F and p′:

$\begin{matrix}{F_{eff} = {\pi\;\Gamma^{2}{C( \frac{M}{2\pi\;{RT}} )}^{\frac{1}{2}}( {p_{1} - p^{\prime}} )}} & ( {{eqn}.\mspace{14mu} 5} ) \\{F_{eff} = {\frac{{\pi\Gamma}^{4}}{16\mspace{11mu}{µL}}( {p^{\prime\; 2} - p_{2}^{2}} )( {1 + {4( {\frac{2}{f_{d}} - 1} )\frac{\lambda}{\Gamma}}} )( \frac{M}{RT} )}} & ( {{eqn}.\mspace{14mu} 6} )\end{matrix}$where μ is the viscosity, f_(d) is the fraction of molecules diffuselyreflected from the walls (0.85<f<1), and C is a constant (C=20).

After determining the mass flow rate, F_(eff), through the orifice itbecomes necessary to describe the flux intensity profile of the effusingbeam, that is, to determine f=f(r, θ), where f is the flux, r is thedistance from the effusion orifice, and θ is the azimuthal angle. Anequation describing the flux as a function of θ and the rate of effusionis obtained by setting the rate of effusion equal to the integral of theflux over a hemispherical area. Assuming that the flux can beapproximated by f=a cos^(n) θ,

$\begin{matrix}{F_{eff} = {\int_{0}^{\frac{\pi}{2}}{\int_{0}^{2\pi}{a\mspace{11mu}\cos^{n}{\theta( {r^{2}\;\sin\mspace{11mu}\theta} )}{\partial\xi}{\partial\theta}}}}} & ( {{eqn}.\mspace{14mu} 7} )\end{matrix}$After solving for a in eqn. 7,

$\begin{matrix}{f = {\frac{F_{eff}( {n + 1} )}{2\pi\; r^{2}}\cos^{n}\theta}} & ( {{eqn}.\mspace{14mu} 8} )\end{matrix}$Although the a priori prediction of a value for n is not completely welldefined, a safe approximation for both transitional flow regimes andfree molecular regimes of L/D=1 is n=2. Alternatively, the exponent inthe flux distribution function (n) may equal 3 or more.

Effusion rate from a given nozzle is a function of vapor pressure withinthe inside of the associated crucible, and this pressure is a functionof the temperature of the molten material inside the reservoir in thatcrucible. Thus, for a particular selected nozzle size, the effusion rateto be expected is essentially a function of the temperature within thecrucible. Predicting the rates of effusion of the copper, gallium, andindium sources is a straightforward solution of the equations above. Thetemperature-vapor pressure data of the three elements are easily foundin literature and can be approximated by:Cu: log P _(Cu) ^(sat)=−19.818+2.0643×10⁻² ×T−5.2119×10⁻⁶ ×T ²  (eqn. 9)Ga: log P _(Ga) ^(sat)=−17.2982+2.0829×10⁻² ×T−6.0×10⁻⁶ ×T ²  (eqn. 10)In: log P _(In) ^(sat)=−16.238+2.1427×10⁻² ×T−6.7885×10⁻⁶ ×T ²  (eqn.11)where pressure is in torr and temperature is in ° C.

Application of the above principals reveals that the vapor flux incidentat the deposition surface presented in a deposition chamber such aschamber 24 is, essentially, a function of the temperature within aselected crucible (and more specifically at each effusion port), thedistance between the effusion ports and the intended deposition surface,and the angle between a point on the substrate and each respectiveeffusion port. Accordingly, for a given configuration of ports and asubstrate web traveling through the deposition region at a constantspeed, the amount of metal vapor (collectively) incident at thedeposition surface of the traveling substrate material is essentially afunction of the temperature of the molten materials within thecrucibles. Thus, by carefully controlling the temperatures of the moltenmaterials at each effusion port, and by maintaining a substantiallyconstant transport speed of the substrate material through thedeposition region, the rate at which metal vapor from each crucible isapplied to the appropriate deposition surface of the traveling substratecan be controlled readily to produce uniform thin-layer depositionthickness along the length of such material.

Variations in the thermal properties of insulation or heating elements,and even convective currents in the “melt” of the evaporating material,may cause temperatures near the effusion ports to differ, and thusaffect the flow rate. Furthermore, the temperature inside an effusionsource near each effusion port becomes increasingly difficult to controlas the source increases in length, and the physical separation betweenthe most distant effusion ports becomes large compared to the otherdimensions of the source. However, wider webs typically require longersources to coat the entire web width, posing a flow rate controlproblem. As described in more detail in Section III below, the presentteachings seek to minimize these difficulties by using multiple, shortersources, and by allowing for flow rate adjustment of each sourceindependently to maintain a desired overall effusion rate.

III. Multi-Zone Deposition

This section relates to systems and methods for depositing a thin-filmp-type semiconductor layer onto a substrate in a specific exemplarymulti-zone deposition process. As described previously and depictedschematically in FIG. 2, a semiconductor layer generally may bedeposited sequentially, by applying various components of the layerseparately and/or in overlapping combinations. FIG. 3 is a more detailedschematic side elevational view of an apparatus for performing such asequential deposition process. As FIG. 3 depicts, the deposition may beaccomplished in a seven-zone procedure, wherein six of the seven zonesare used to deposit portions of the semiconductor layer, and a seventhintermediate zone is used to monitor one or more properties of thepreviously deposited layers. The seven-zone procedure depicted in FIG. 3and described herein is exemplary, and it should be appreciated that aneffective p-type semiconductor layer may be deposited in a similarprocedure having greater or fewer than seven zones.

In the exemplary procedure of FIG. 3, as in the more general proceduredepicted in FIG. 2, deposition of the semiconductor layer occurs insidea deposition region R of an absorber layer deposition chamber 100 thathas been evacuated to near vacuum, typically to a pressure ofapproximately 0.7-2.0 millitorr (700-2000 μTorr) that is provided byselenium gas. Also as in the general embodiment of FIG. 2, deposition inthe embodiment of FIG. 3 proceeds via a roll-to-roll,molten-liquid-to-vapor co-evaporation process, wherein a substrate web102 is transported through the deposition region from a pay-out roll 104to a take-up roll 106, with the pay-out roll and the take-up roll bothlocated within deposition chamber 100. Alternatively, the pay-out andtake-up rolls may be disposed outside of, but in close proximity to, thedeposition chamber. Substrate heaters 103 may be positioned at one ormore locations of the processing path to heat substrate web 102.

Each of the six deposition zones described in this section may have asimilar basic structure but may vary as to number, deposition materialand location within the zone, of material sources. Each zone may includeat least two material sources, for example the material sources shown inFIG. 4, each configured to emit plumes of molecules to be deposited onthe moving substrate web 102, which passes above and at a distance fromthe sources. Two of the at least two material sources may be disposedsubstantially symmetrically across the transverse dimension or width ofthe web and may contain the same deposition material to be depositeduniformly on the moving substrate web 102.

In some zones, such as in the zone depicted in FIG. 4 and described inmore detail below, two separate deposition materials may be depositedonto the web. In such cases, four sources may be provided, a first setof two sources disposed substantially symmetrically across thetransverse dimension or width of the web containing a first depositionmaterial and a second set of two sources disposed substantiallysymmetrically across the transverse dimension or width of the webcontaining a second deposition material. Each set of two sources may beconfigured to deposit a different material across the entire width ofthe web. In other zones, where only a single material is deposited ontothe web, a single set of two sources may be provided and configured todeposit one material across the web.

Each deposition zone may be enclosed within a separate solid enclosure101. Generally, each enclosure 101 may surround the associateddeposition zone substantially completely, except for an aperture in thetop portion of the enclosure over which the moving substrate web passes.This allows separation of the deposition zones from each other,providing the best possible control over parameters such as temperatureand selenium pressure within each zone.

The exemplary chamber 100 of FIG. 3 is designed specifically for thecreation of a CIGS layer by passing the substrate web through sevenseparate zones, including at least one or more deposition zones, withindeposition region R, resulting in a CIGS layer of composite thicknessbetween a few hundred and a few thousand nanometers. Provided bellow isa sequential description of each of the seven zones (110, 112, 126, 128,132, 134, and 136) shown in FIG. 3.

Specifically, first zone 110 may be configured to deposit a layer ofsodium fluoride (NaFI) onto the web. The presence of sodium is believedto improve p-type carrier concentration by compensating for defects inone or more of the subsequently deposited CIGS layers, and thus toimprove the overall efficiency of the PV cell. An initial layer of NaFIhas been found to be optimal. Alternatively, potassium (K) or lithium(Li) may serve a similar purpose as sodium. Furthermore, other compoundsaside from NaFI, such as sodium selenide (Na₂Se₂), sodium selenite(Na₂SeO₃), sodium selenate (Na₂O₄Se), or other similar compoundsincorporating potassium and/or lithium, also may be suitable forimproving p-type carrier concentration.

Second zone 112, which is shown in isolation in FIG. 4, may beconfigured to deposit a layer of gallium indium (GI) onto the web (ormore precisely, onto the previously deposited layer of NaFI). Secondzone 112 may include two gallium sources 114 disposed substantiallysymmetrically across the transverse dimension of the web and two indiumsources 116 similarly disposed substantially symmetrically across thetransverse dimension of the web. Also depicted in second zone 112 ofFIG. 4 is a selenium (Se) source, generally indicated at 118. Seleniumsource 118 is configured to provide selenium gas to second zone 112.Providing a background of selenium gas results in deposition on thesubstrate web of selenium along with the GI layer.

GI (more specifically GI selenide) may be deposited through the nearlysimultaneous—but separate—deposition of gallium and indium onto the sameportion of the moving web. As indicated in FIG. 4, however, galliumsources 114 may be located slightly before indium sources 116 within thesecond zone 112, so that a small amount of gallium is deposited onto theweb prior to deposition of any indium. Because gallium adheres better tothe underlying web and to the previously deposited NaFI molecules, thisarrangement results in better overall adhesion of the GI layer depositedin the second zone.

Selenium source 118 is configured to provide selenium gas to second zone112, and similar selenium sources may also be located in the third,fifth, sixth and/or seventh zones within chamber 100 to provide seleniumgas to the third, fifth, sixth and/or seventh zones within chamber 100,up to a pressure in the range of approximately 700-2000 μTorr. Eachselenium source in a zone may be independently monitored and controlled.Providing a background of selenium gas results in deposition of seleniumalong with the other source materials, such as GI, such that thedeposited layer may comprise indium-gallium selenide, gallium selenideor gallium-rich indium-gallium selenide.

As shown in more detail in FIG. 4, each of the two gallium sources 114and each of the two indium sources 116 within second zone 112, and moregenerally each material source in any of the zones of chamber 100, maygenerally include a crucible or body portion 120, and a lid 122containing one or more effusion ports 124.

Each deposition zone may itself be enclosed within a separate solidenclosure 101. Generally, each enclosure 101 may surround the associateddeposition zone, for example second zone 112, substantially completely,except for an aperture 101 a in the top portion of enclosure 101, overwhich the moving substrate web passes. This allows separation of thedeposition zones from each other, providing the best possible controlover parameters such as temperature and selenium pressure within eachzone. Aperture 101 a in the top portion of enclosure 101 may have awidth that is substantially the same as the width of substrate web 102.

A deposition material is liquefied or otherwise disposed within the bodyportion 120 of a given source, and emitted at a controlled temperaturein plumes of evaporated material through effusion ports 124. Asdescribed previously, because the angular flux of material emitted froman effusion port 124 with a particular geometry is a function primarilyof temperature of the port and/or deposition material, this allows forcontrol over the thickness and uniformity of the deposited layerscreated by the vapor plumes.

As shown in FIG. 3, third deposition zone 126 may be configured todeposit a layer of copper (Cu) onto the moving web. Third depositionzone 126 may include two material sources, which are structurallysimilar or identical in construction to the gallium and indium sources114 and 116 described with reference to FIG. 4. Specifically, thirddeposition zone 126 may include two material sources, containing thedeposition material copper, disposed substantially symmetrically acrossthe transverse dimension or width W of the web. The two sources maygenerally include at least a body portion, and a lid containing one ormore effusion ports. Third deposition zone 126 may also include aselenium source.

Sources of copper material may disposed within the third zone 126relatively close to the entrant side of the substrate web 102 into thethird zone 126, but alternatively may be disposed more toward the egressside of the third zone 126 with similar effect. However, by providingthe copper sources relatively close to the entrant side of the thirdzone 126, the copper atoms have slightly more time to diffuse throughthe underlying layers prior to deposition of subsequent layers, and thismay lead to preferable electronic properties of the final CIGS layer.

Fourth zone 128 may be configured as a sensing zone, in which one ormore sensors, generally indicated at 130, monitor the thickness,uniformity, or other properties of some or all of the previouslydeposited material layers. Typically, such sensors may be used tomonitor and control the effective thickness of the previously depositedcopper, indium and gallium on the web, by adjusting the temperature ofthe appropriate deposition sources in the downstream zones and/or theupstream zones in response to variations in detected thickness. Tomonitor properties of the web across its entire width, two or moresensors may be used, corresponding to the two or more sources of eachapplied material that span the width of the web disposed substantiallysymmetrically across the transverse dimension of the web. Fourth zone128 is described in more detail below with reference to FIG. 6.

Fifth zone 132 may be configured to deposit a second layer of copper,which may have somewhat lesser thickness than the copper layer depositedin third zone 126, from a pair of sources disposed substantiallysymmetrically across the transverse dimension of the web. Similar to thecopper sources described in third zone 126, two copper sources withinfifth zone 132 may be configured to emit copper plumes from multipleeffusion ports spanning the width of the substrate web. Furthermore, thecopper sources may be disposed on the entrant side of the fifth zone 132to allow relatively more time between copper deposition and subsequentlayer deposition. Fifth zone 132 may also include a selenium source.

Sixth zone 134 may be configured to deposit a second layer ofgallium-indium onto the web. In construction, sixth zone 134 may besimilar to second zone 112. The thickness of the gallium-indium layerdeposited in sixth zone 134 may be small relative to the thickness ofthe GI layer deposited in second zone 112. In sixth zone 134, galliumand indium may be emitted at somewhat lesser effusion temperaturesrelative to the effusion temperatures of the gallium and indium emittedin second zone 112. These relatively lower temperatures result in lowereffusion rates, and thus to a relatively thinner layer of depositedmaterial. Such relatively low effusion rates may allow fine control overratios such as the copper to gallium+indium ratio (Cu:Ga+In) and thegallium to gallium+indium ratio (Ga:Ga+In) near the p-n junction, eachof which can effect the electronic properties of the resulting PV cell.As in the second zone 112, gallium may be emitted slightly earlier alongthe web path than indium, to promote better adhesion to the underlyinglayers of molecules.

Seventh zone 136 may be similar in construction to one or both of secondzone 112 and sixth zone 134 and may be configured to deposit a thirdslow-growth, high quality layer of gallium-indium (GI) onto thesubstrate web. In some embodiments, this final deposition zone and/or GIlayer may be omitted from the deposition process, or a layer of indiumalone may be deposited in seventh zone 136. As in sixth zone 134,application of a relatively thin, carefully controlled layer of galliumand/or indium allows control over ratios such as (Cu:Ga+In) and(Ga:Ga+In) near the p-n junction. This may have a beneficial impact onthe efficiency of the cell by, for example, allowing fine-tuning of theelectronic band gap throughout the thickness of the CIGS layer.Furthermore, the final layer of GI is the last layer applied to completeformation of the p-type CIGS semiconductor, and it has been foundbeneficial to form a thin layer of GI having a relatively low defectdensity adjacent to the p-n junction that will be subsequently formedupon further application of an n-type semiconductor layer on top of theCIGS layer.

As shown in FIG. 4, second zone 112 may include two gallium sources 114disposed substantially symmetrically across the transverse dimension ofthe web, and two indium sources 116 disposed substantially symmetricallyacross the transverse dimension of the web. In other words, two sourcescontaining identical deposition material may span the width of substrateweb 102, to provide a layer of material across the entire width of theweb having a uniform thickness. The operation, including effusion rateand/or temperature of each source in a zone may be controlled and/ormonitored independently of the second source in the zone having the samedeposition material. For example, each gallium source 114 a may includea heating element that is adjustable independent of a heating elementincluded in the second gallium source 114 b.

This basic structure, with at least two independently operable heatedsources containing the same deposition material spanning the web width,may be common to each of the zones of chamber 100 in which material isdeposited onto the web (deposition zones 110, 112, 126, 132, 134 and136). By providing two independent sources of material disposedsubstantially symmetrically across the width of the web, the thicknessof each deposited material may be independently monitored on each sideof the web, and the temperature of each source may be independentlyadjusted in response. This allows a wider web to be used, leading to acorresponding gain in processing speed per unit area, withoutcompromising material thickness uniformity.

Turning now to FIG. 5, sensing zone 128 may include sensors configuredto monitor the layers of material deposited in one or more of the zones110, 112 and 126. Exemplary types of sensors may include one or more ofX-Ray Florescence, Atomic Absorption Spectroscopy (AAS), ParallelDiffraction Spectroscopic Ellipsometry (PDSE), IR reflectometry,Electron Impact Emission Spectroscopy (EIES), in-situ x-ray diffraction(XRD) both glancing angle and conventional, in-situ time-resolvedphotoluminescence (TRPL), in-situ spectroscopic reflectometry, in-situKelvin Probe for surface potential, and in-situ monitoring of emissivityfor process endpoint detection. Sensing zone 128 may further include asensing shield such as an angled polyimide sensor shield. Sensing zone128 may use an H₂O cooled enclosure. As shown in FIG. 6, sensing zone128 has a monitoring station 130 configured to collect data indicatingproperties, for example, relating to layer uniformity, of materialdeposited on web 102. In a preferred embodiment, monitoring station 130contains multiple sensors 130 a and 130 b across the width of the web,corresponding to the set of two independently controllable sources ofmaterial that are disposed substantially symmetrically across the widthof the substrate web in each deposition zone, provides the ability tomonitor and control layer uniformity across the width of the web.

Specifically, monitoring station 130 in sensing zone 128 may includesensors 130 a and 130 b. One or more computers, 131, may be configuredto analyze data from the monitoring station to monitor a property, suchas thickness, of one or more of the deposited layers, and subsequentlyadjust the effusion rates and/or temperatures of a corresponding sourcein deposition zones 110, 112, and/or 126. Additionally and/oralternatively, the effusion rates and/or temperatures of a correspondingsource in downstream deposition zones 132, 134, and/or 136 may beadjusted in view of a property monitored by a sensor in sensing zone128.

Additionally and/or alternatively, a second similar monitoring stationjust prior to take-up roller 106 may be used. Similar to monitoringstation 130 in zone 128, monitoring station 140 may include two sensorsprovided across the width of the web, corresponding to the twoindependently controllable sources of each material that are disposedsubstantially symmetrically across the width of the web in eachdeposition zone. Monitoring station 140 monitors one or more propertiesof the layers of material deposited in one or more of zones 132, 134 and136 and/or zones 110, 112 and 126. Monitoring station 130 may monitor aproperty of the gallium-indium and copper material layers deposited inzones 112 and 126, while monitoring station 140 cooperatively monitorsone or more properties of the copper and gallium-indium material layersdeposited in zones 132 and 134.

The flow chart in FIG. 6 depicts an exemplary method of depositing athin film semiconductor layer on a substrate including feedback systemsin accordance with the present disclosure. As described earlier,properties, such as thickness, of deposited layers are at leastpartially dependant on effusion rates of the sources containing thedeposition material. The effusion rate of a source may be altered bychanging the temperature of the source. A system including one or morefeedback systems may be configured to adjust the effusion rate of asource in response to a monitored property of a deposited layer ofmaterial, for example by adjusting the temperature of the source. Themethod shown in FIG. 6 may be used to produce a more consistentlyuniform deposition of material layers.

FIG. 6 shows a preferred method and procedure 150 for depositingthin-film semiconductor materials. A web material is fed through aseries of deposition zones and sensing zones as previously described andillustrated. A first step 152 in the process involves depositing analkaline metal, for example, in the form of sodium chloride on the web.In a second step 154, a layer of gallium and indium is deposited in thepresence of selenium gas. In a third step 156, a copper layer isdeposited on top of the gallium indium layer, in the presence ofselenium gas. A next step monitoring 158 monitors and controls thequality of the layers being created in the first two steps. Datagenerated in monitoring step 158 may be used and/or processed bycontroller 160 to provide feedback control and/or correction/adjustmentof steps 154 and 156 as the process continues.

After monitoring step 158, step 162 carries out deposition of a secondcopper layer in the presence of selenium gas. Step 162 is followed by aseries of two steps 164 and 166 of gallium and indium, in the presenceof selenium gas. As shown in FIG. 6, a final monitoring step 168 iscarried out to provide further monitoring and control capability, viacontroller 160, particularly with respect to the layers being created insteps 162, 164 and 166.

FIG. 7 shows a system for depositing a thin film semiconductor layerincluding a transport-guide structure defining a processing path for aflexible substrate. Transport-guide structure 200 may be configured tomaintain a desired tension on substrate web 204. The desired tension maydepend on the width of the substrate web, for example a 30 cm widesubstrate web may have a desired tension of approximately 30-35 lbs.Maintaining tension on the web may reduces curling of the web and/orhelps to maintain a substantially flat surface for deposition ofevaporated materials in the deposition zones.

A transport-guide processing path may wind around a number of rollers ina generally curved, non-planar overall configuration. The substrate webmay at least partially wrap around transport rollers in zone transitionareas to maintain substrate web flatness and proper web guiding betweeneach zone. The wrap angle may create marginally enough friction to makethe free roller rotate at the same speed as the substrate web travelspeed. The transport rollers may be non-driven and may rotate freely atthe same speed as substrate web travel. If the rollers do not move atthe same speed as the traveling substrate web, the substrate web mayslide across 1 or more rollers, scratching the back surface of the web.These scratches ‘print-thru’ to the front side, causing defects in theCIGS coating that reduce solar cell efficiency.

In accordance with the present disclosure, transport-guide structure 200includes pay-out roll 202 configured to supply flexible substrate web204 into deposition region R and take-up roll 206 is configured toreceive substrate web 204 at the end of the process. Pay-out roll 202and take-up roll 206 may be driven by independent motors. Transportrollers 208 may be located at spaced intervals along the processing pathof substrate web 204. The spaced intervals are generally uniform inlength and/or may include one or more deposition or sensing zones.

Placement of transport rollers within deposition chamber 200 may be suchthat a pair of adjacent transport rollers may be configured to orientthe substrate web within a spaced interval or zone at a particular anglerelative to the horizontal. The angle of the substrate material relativeto the horizontal may vary between each zone as a function of placementof the transport rollers. In the embodiment shown, similar to FIG. 3,deposition chamber 200 includes seven zones. The angle of the substratematerial relative to the horizontal may progressively vary by a degreesbetween each zone. The progressive change α may be the same, for exampleα1-α6 may equal approximately 7 degrees per roller (per turn).Alternatively, the degree of change between each zone α1-α6 may vary.The angle of the substrate material relative to the horizontal may notbe so great, however, that evaporated material can not be depositeduniformly on the substrate.

Transport roller material may include stainless steel such as Type 304stainless. The shape of the transport rollers themselves may beconfigured to ensure a central flat surface area on the substrate todeposit evaporated materials. Slightly tapered rollers can maintain webtension and a central flat surface area by distributing tension on theweb in a way that ‘pulls’ the web toward the roller edges from thecenterline and helps keep the web flat. For example, the taper may becreated by about a 0.003″ curvature at the edges of a 3″ diameterroller.

IV. Sources

This section describes methods and apparatus for controlling thetemperature of the effusion ports that emit vapor plumes to be depositedon the substrate web during the p-type semiconductor deposition process.

As described previously and depicted schematically in FIG. 2, asemiconductor layer generally may be deposited onto a substrate websequentially, by applying various components of the layer separatelyand/or in overlapping combinations. Whereas FIG. 2 depicts thedeposition occurring within a single deposition chamber 24,alternatively, deposition may occur within a series of separated (i.e.discrete) deposition zones, with one or more materials deposited withineach zone. FIG. 4 is a perspective view showing an exemplary discretedeposition zone 112 for depositing one or more semiconductor materialsonto a substrate web 102. FIG. 4 shows zone 112 with its surroundingenclosure in dashed lines.

Zone 112 of FIG. 4 is configured to deposit a layer of gallium-indium(GI) selenide onto substrate web 102 (including any previously depositedlayers of material). GI is deposited through the nearly simultaneous—butseparate—deposition of gallium and indium onto the same portion of themoving web. As indicated in FIG. 4, however, gallium sources 114 arelocated slightly before indium sources 116 within the deposition zone,so that a small amount of gallium is deposited onto the web prior todeposition of any indium. Selenium source 118 may be located before thegallium sources 114 and may be configured to provide selenium gas tosecond zone 112, up to a pressure in the range of approximately 700-2000μTorr. Providing a background of selenium gas results in deposition ofselenide along with the GI layer. Because gallium or gallium selenideadheres better to the underlying web, this arrangement results in betteroverall adhesion of the GI layer deposited in the second zone.

Each gallium and indium source within a zone includes a crucible or bodyportion, and a lid containing one or more effusion ports. A depositionmaterial is liquefied or otherwise disposed within the body portion of agiven source, and emitted at a controlled temperature in plumes througheffusion ports. Each zone may itself be enclosed within a separate solidenclosure. Generally, each enclosure will surround the associateddeposition zone substantially completely, except for an aperture in thetop portion of the enclosure over which the moving substrate web passes.This allows separation of the deposition zones from each other,providing the best possible control over parameters such as temperatureand selenium pressure within each zone. A deposition material isliquefied or otherwise disposed within the body portion of a givensource, and emitted at a controlled temperature in plumes through theeffusion ports. As described previously, because the angular flux ofmaterial emitted from a port with a particular geometry is a functionprimarily of temperature, this allows for control over the thickness anduniformity of the deposited layers created by the vapor plumes.

FIGS. 8-13 depict closer views of portions of a heated effusion source300, examples of which include any one of effusion sources 114 or 116,for generating a vapor plume of material to be deposited onto asubstrate web. Although these sources have been described previously assuitable for generating plumes of gallium or indium, the plume may becomposed more generally, for example, of sodium fluoride, gallium,indium, copper, or a combination of two or more of these materials.

As in the case of sources 114 and 116, source 300 has a body portion302, and a lid 304 with integral effusion ports 306. Each effusion port306 may be formed by two complimentary looped portions of a heatingelement 308 integral with lid 304. In such a configuration, lid 304 mayalso be referred to as a “heater plate.” Heating element 308 mayradiated directly toward the source material in body portion 302 orheating element 308 may radiate heat to lid 304, which the re-radiatesto the source material. Heating element 308 may be fully containedwithin lid 304 or alternatively a portion or all of heating element 308may be exposed and/or extend towards the source material and/depositionregion R.

As best seen in FIG. 10, heating element 308 may have a base height 308a and a nozzle or port height 308 b extending above the base height 308a. The base height 308 a and the port height 308 b may together form thewalls of the effusion ports 306. The port height 308 b may extend about6 mm (2-25 mm) above base height 308 a. The surface area of the bottomface of heating element 308 may be approximately 23.87 cm². Lid 304 maybe 112.69 cm² in area (on the upper face) and 112.69 cm² area normal tothe horizontal plane for the lower face.

Lid 304 materials may include, for example, graphite, such as ET10graphite manufactured by the Ibiden Corporation of Elgin, Illinois. Thisunderlying graphite may be coated with a material, such as pyroliticboron nitride, designed to withstand the extreme conditions associatedwith heated effusion. The lid material is electrically conducting, sothat lid 304 can function as a self-contained heater as well as a lidwith integrated effusion ports. Insulation included in lid 304 maysurround at least the outside of effusion ports 126 above the baseheight of heating element 308 in the form of insulation layers 303,which may include graphite or carbon felt.

Insulation layers 303 may also include a top layer 303 a of thingraphite foil. Effusion ports 306 may be configured to retain the topmost portion of the lid and/or insulation below the top most portion ofthe effusion port. If any portion of insulation or lid should extendabove the effusion port opening, then effusing metal accumulates andcondenses rapidly around the protrusion. To avoid condensation ofeffusing material, the final solid element that may be exposed to theeffusing metal vapor stream must be the heated effusion port.Specifically, the outside of at least one wall of effusion ports 306 mayinclude a lip 314 configured to retain thin graphite foil layer 303 ajust below the top level of effusion port 306. Thin foil layer 303 a inturn keeps the necessary insulation (such as graphite felt) around theheated ports below the top level of the port.

To supply power to the lid/heater, a pair of electrical contacts 210 mayextend in toward the central portion of the lid. These contactsgenerally are configured to apply a voltage across the lid, creating anelectrical current that heats the lid resistively. Thus, by controllingthe applied voltage and/or current, the lid, including the effusionports, may be heated to any desired temperature. By applying electricalcontacts to the symmetric center of the lid (rather than to one side),resistive loss effects also should be symmetric, resulting insubstantially equal temperatures for both effusion ports.

Each effusion port 306 may be formed by two complimentary loopedportions of heating element 308. Each port may be spaced equidistantlyfrom a pair of electrical contacts 210 to obtain substantially equaltemperatures for both ports. The curve of the looped portion may beconfigured to avoid brittleness or cracking of the heating element, forexample the curve may be gradual or C-shaped. The looped portions of theheating element may be electrically insulated from each other by anelectrically insulated gap 212 including a dielectric material. Theheight and shape of effusion ports 306 formed by heating element 308 maybe configured to uniformly deposit evaporated material on a movingsubstrate.

The structure illustrated in FIGS. 8-13 may have a number of desirablefeatures. By incorporating a heater within the lid, direct control isobtained over the temperature of effusion ports 306. In comparison tosystems in which a separate heater (for example, surrounding orotherwise in close proximity to the effusion ports) is used, the presentconfiguration therefore may result in faster response times and moreaccurate temperature control. In addition, the illustrated structureincludes effusion ports 306 that are carefully designed to producematerial plumes that coat an overlying substrate web evenly from side toside. This is accomplished both by a careful choice of effusion portdimensions, and also by spacing the ports at appropriately chosendistances from the source edges and from each other. The choice ofexactly two effusion ports, rather than three or more, also may resultin substantially equal temperatures of the ports due to equal edgeeffects within each source. Furthermore, the integral port/heatercombination maintains high temperatures at the port surfaces exposed tothe effusing flux, thus preventing evaporate condensation and subsequent“spitting,” which negatively effects system quality.

FIGS. 11-13 illustrate different embodiments of a system including atleast two heated sources disposed substantially symmetrically across thewidth of the substrate web. As shown in FIGS. 11-13, the edges of thesubstrate web may extend to approximately outer edges of the outer mosteffusion ports. In other embodiments, the edges of the substrate web mayextend beyond the outer edges of the outermost effusion ports. In suchembodiments the edges of the substrate web may be cut off duringsubsequent processing.

Also illustrated in FIGS. 11-13 is an exemplary distribution of an arrayof effusion ports across the width of a processing path, indicated bydirectional arrows. A uniform distribution of the effusion port arraymay result in a greater concentration of material in the center mostregion of the processing path than in the outside or lateral regions.Accordingly, the distribution of the effusion port array across thewidth of the processing path may be configured to obtain uniform coatingthickness on a substrate web for a source-web distance. In someinstances, the spacing between the center-most effusion ports of thearray in the center of the processing path should be greater than thespacing between effusion ports on the outside of the array in thelateral regions of the processing path. This configuration produces amore uniformly coated substrate web. Nozzle spacing distances may becalculated for optimal layer uniformity by using vapor densityrelationships from a single nozzle orifice, and then assumingsuperposition of multiple plumes from multiple ports to yield totalexpected metal thickness over a given deposition area.

This application also incorporates by reference in their entireties thefollowing patents: Reissue No. Re 31,968, 5,441,897, 5,356,839,5,436,204, and 5,031,229.

The disclosure set forth above may encompass multiple distinctinventions with independent utility. Although each of these inventionshas been disclosed in its preferred form(s), the specific embodimentsthereof as disclosed and illustrated herein are not to be considered ina limiting sense, because numerous variations are possible. The subjectmatter of the inventions includes all novel and nonobvious combinationsand subcombinations of the various elements, features, functions, and/orproperties disclosed herein. The following claims particularly point outcertain combinations and subcombinations regarded as novel andnonobvious. Inventions embodied in other combinations andsubcombinations of features, functions, elements, and/or properties maybe claimed in applications claiming priority from this or a relatedapplication. Such claims, whether directed to a different invention orto the same invention, and whether broader, narrower, equal, ordifferent in scope to the original numbered paragraphs that follow, alsoare regarded as included within the subject matter of the inventions ofthe present disclosure. For various thin layer deposition applications,different combinations of deposition steps and zones may be used inaddition to the specific deposition zone configurations described aboveand in the following claims. None of the particular steps included inthe examples described and illustrated are essential for everyapplication. The order of steps/zones, combination of steps/zones, andthe number of steps/zones may be varied for different purposes. It maysometimes be useful to increase the number of deposition zones whiledecreasing the amount of material deposited at each zone. It may also beuseful to increase the number of sources used to deposit material overthe width of a web, with or without multiple corresponding monitoringdevices to carry out on-the-fly adjustment of heater temperatures andresulting improvements in thin layer uniformity across the web. Othervariables may be controlled via the described monitoring stations, forexample speed of web transport, pressure, selenium gas output, webtemperature, etc.

1. A method of depositing a thin film semiconductor layer onto aflexible substrate, comprising: transporting a substrate web through amulti-zone deposition region, depositing onto the web, within a firstsubstantially enclosed gallium indium deposition zone, a first layer ofgallium indium in the presence of selenium gas, depositing onto the web,after depositing the first layer of gallium indium and within a firstsubstantially enclosed copper deposition zone, a first layer of copperin the presence of selenium gas, depositing onto the web, afterdepositing the first layer of copper and within a second substantiallyenclosed copper deposition zone, a second layer of copper in thepresence of selenium gas, and depositing onto the web, after depositingthe second layer of copper and within a second substantially enclosedgallium indium deposition zone, a second layer of gallium indium in thepresence of selenium gas.
 2. The method of claim 1, further comprisingdepositing onto the web, prior to depositing the first layer of galliumindium, a layer of precursor material selected from the group consistingof sodium fluoride, potassium fluoride, lithium fluoride, sodiumselenide, sodium selenite and sodium selenate.
 3. The method of claim 2,further comprising depositing onto the web, after depositing the secondlayer of copper and within a third substantially enclosed gallium indiumdeposition zone, a third layer of gallium indium in the presence ofselenium gas.
 4. The method of claim 3, further comprising monitoring aproperty of a layer deposited on the web between the steps of depositingthe first and second layers of copper.
 5. The method of claim 4, furthercomprising adjusting an amount of material deposited in the first layerof gallium indium or first layer of copper based on data developed inthe monitoring step.
 6. The method of claim 5, wherein the monitoringstep is carried out using x-ray fluorescence.
 7. The method of claim 6,wherein the adjusting step includes altering a heating temperature inone of the first two depositing steps.
 8. The method of claim 7, whereineach depositing step includes effusing material from side-by-sidesources, the monitoring and adjusting steps being configured to correctlayer non-uniformity across the web.
 9. The method of claim 5, whereinthe monitoring and adjusting steps are carried out while the web movescontinuously through the multi-zone deposition region.
 10. The method ofclaim 4, further comprising monitoring a property of a layer depositedon the web after depositing the third layer of gallium indium.
 11. Themethod of claim 10, further comprising adjusting an amount of materialdeposited in one or more of the second layer of copper, the second layerof gallium indium, and the third layer of gallium indium based on datadeveloped in the monitoring step.
 12. The method of claim 1, furthercomprising monitoring a property of the thin film semiconductor layerafter depositing the first layer of copper and prior to depositing thesecond layer of copper, and adjusting a deposition rate of at least oneof the first layer of gallium indium and the first layer of copper basedon the monitored property.
 13. The method of claim 1, further comprisingdepositing onto the web, after depositing the second layer of galliumindium and within a third substantially enclosed gallium indiumdeposition zone, a third layer of gallium indium in the presence ofselenium gas.